The invention relates to an exhaust-gas turbocharger for an internal combustion engine having a turbine housing in which a turbine wheel with a shaft is rotatably arranged, wherein the shaft is rotatably mounted in a bearing housing by way of at least one piston ring adjacent to the turbine wheel, and the bearing housing has, for the lubrication of the shaft, a lubricant inlet and an unpressurized lubricant outlet.
Exhaust-gas turbochargers have a dual housing in which, at one side, a turbine wheel and, at the other side, a compressor wheel on a shaft are rotatably mounted by way of radial bearings in a bearing housing. Here, the shaft may reach rotational speeds of, for example, up to 300,000 rpm. To accommodate axial forces that arise, it is also possible for at least one axial bearing to be provided. Here, the bearings are lubricated by a corresponding lubricant.
In the case of the exhaust-gas turbocharger, it is possible, owing to the oil-lubricated bearing arrangement thereof, for lubricant losses to occur, in particular into the turbine housing. For the exhaust-gas turbocharger, dynamic sealing is often realized by use of piston rings, though these cannot suitably prevent a passage of lubricant to the turbine side under all operating conditions. Owing to modern exhaust-gas technology, a passage of lubricant, in particular to the turbine side, must be avoided as far as possible, because this leads to sooting (deposition of tar-like substances) of the turbine wheel and/or to sooting of the catalytic converter, the effectiveness of which with regard to exhaust-gas purification is thereby severely impaired.
The German laid-open specification DE 10 2007 027 869 A1 discloses a turbocharger having a turbocharger housing. A leadthrough opening is provided for a shaft, wherein the shaft is rotatably mounted in the housing by way of a bearing arrangement. Lubricating oil is supplied to the bearing for lubrication purposes. A section of the shaft is provided at the outer side of at least one bearing, which section forms a gap with the housing. The gap is in the form of a lubricating oil throttle in order to at least reduce a passage of lubricating oil from the bearing side into the turbine housing.
A disadvantage of said known prior art is that a passage of lubricating oil via the bearings to the turbine side is merely reduced, and not prevented.
It is an object of the present invention to provide a measure to further reduce, or prevent, a passage of lubricant into the turbine housing.
This and other objects are achieved by an exhaust-gas turbocharger having a turbine housing in which a turbine wheel with a shaft is rotatably arranged, wherein the shaft is rotatably mounted in a bearing housing by way of at least one piston ring adjacent to the turbine wheel, and the bearing housing has, for the lubrication of the shaft, a lubricant inlet and an unpressurized lubricant outlet. Between the turbine wheel and the bearing housing, a thermal disk is arranged which is clamped by the turbine housing and the bearing housing and which has an opening connected in lubricant-conducting fashion to the lubricant outlet
Lubricant which, by cohesion, is conveyed past the piston rings to the outside is advantageously returned to the lubricant circuit through the opening in the thermal disk and the corresponding bore in the bearing housing. This leads to a “dry” turbine housing and to a considerable reduction of lubricant residues (sooting) in the region of the piston rings.
An advantageous refinement of the invention provides a lubrication gap between the shaft and the bearing housing adjacent to the piston ring, which is connected in lubricant-conducting fashion to the lubricant outlet.
The lubricant which creeps via the piston rings is advantageously returned via the bore in the bearing housing and thus to the lubricant outlet.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.